diff --git a/src/inc/notifications/NotificationSlack.class.php b/src/inc/notifications/NotificationSlack.class.php
index 75fe29770..67c85afce 100644
--- a/src/inc/notifications/NotificationSlack.class.php
+++ b/src/inc/notifications/NotificationSlack.class.php
@@ -36,6 +36,7 @@ function sendMessage($message, $subject = "") {
       curl_setopt($ch, CURLOPT_HTTPPROXYTUNNEL, "true");
     }
     
+    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-type: application/json'));
     curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
     curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
     curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);